This invention relates to motor controls, and in particular, to a method of controlling the starting of an AC induction motor with a soft starter.
There are two basic approaches for controlling the starting, stopping and speed of an AC induction motor. In a first approach, an adjustable frequency controller is interconnected to the AC induction motor. The adjustable frequency controller is comprised of an inverter which uses solid state switches to convert DC power to stepped waveform AC power. A waveform generator produces switching signals for the inverter under control of a microprocessor. While adjustable frequency controllers efficiently control the motor speed and the energy used by an AC induction motor, use of such types of controllers may be cost prohibitive. Further, since many applications of AC induction motors do not require sophisticated frequency and voltage control, an alternative to adjustable frequency controllers has been developed.
An alternate approach to the adjustable frequency controller is the soft starter. Soft starters operate using the principal of phase control whereby the line currents supplied to the AC induction motor are controlled by means of anti-parallel thyristor switches. Soft starters offer two major benefits for a user. First, use of a soft starter reduces the motor torque pulsation at startup of the AC induction motor which, in turn, results in less mechanical strain on the load. Second, use of a soft starter reduces the motor inrush current at startup of the AC induction motor which, in turn, places less stress on upstream electrical systems.
Typically, the anti-parallel thyristor switches are provided in each supply line. However, with delta motors, the anti-parallel thyristor switches may be provided inside the delta. Positioning the anti-parallel thyristor switches within the delta allows for use of smaller electrical components since the phase current magnitudes are less than the line current magnitudes.
In order to start an AC induction motor using a soft starter, two alternate types of control algorithms are used. In alpha control, the thyristor switches are sequentially fired at a certain angle, alpha (xcex1), after the zero crossing times of the corresponding phase voltages. In gamma control, the thyristor switches are sequentially fired at a certain angle, gamma (xcex3), after the zero crossing times of the corresponding phase currents. It has been found that alpha control is more stable than gamma control in starting and bringing the AC induction motor to near operating speed because the zero crossings of the phase voltage are fixed while the zero crossings of the phase current move as the speed of the AC induction motor changes.
In addition, if gamma control is used during such period, greater current oscillation occurs which, in turn, leads to greater torque oscillation. More specifically, torque oscillation when the motor is stalled or when accelerating up to speed generally is caused by a DC component in the motor current. One of the principle advantages of utilizing a soft starter is that by applying the voltage to the motor slowly, these DC offset currents can be eliminated. However, it is possible to re-introduce DC currents when using gamma control. For example, utilizing a constant gamma when the integral of the motor phase current for the positive half cycle differs from the integral of the motor phase current for the negative half cycle, produces a DC component. In fact, if a disturbance introduces a slight asymmetry in firing of the thyristor switches, gamma control will exhibit a tendency to amplify the asymmetry, since an early current zero crossing will cause the next thyristor firing to occur earlier with respect to the terminal voltage. This effect is more pronounced when the soft starter is positioned inside the delta because the thyristor switches directly control phase voltage, that the current through the thyristor switches reflects the actual power of the motor (mostly inductive during acceleration), whereas line current control interposes a 30 degree angle between motor terminal voltage and line current being controlled.
Alternatively, gamma control is preferred once the motor has accelerated to the point where its speed is above the breakdown speed, in other words, the speed at which the slope of the speed/torque curve is zero. This is due to the fact that the motor""s torque production is a delayed function of speed. Therefore, as the motor accelerates to full speed, it overshoots before the electrical torque balances the mechanical load. Once the motor overshoots, the electrical torque eventually becomes negative and the speed undershoots its steady state value causing torque to become positive again. It may take several cycles of this oscillation to reach steady state when the terminal voltage on the motor is fixed. However, when gamma control is used, the changes in motor power factor caused by the speed changes result in changes in current zero cross time which, in turn, affect the firing point for the next half cycle of current. This results in slight cycle to cycle changes in terminal voltage. These changes have a damping effect on the oscillations described above. In fact, in many cases, the oscillations are damped within a complete cycle.
Therefore, it is a primary object and feature of the present invention to provide an improved method for controlling the starting of an AC induction motor.
It is a further object and feature of the present invention to provide a method for controlling the starting of an AC induction in a stable manner.
It is still a further object and feature of the present invention to provide a method for controlling the starting an AC induction motor which utilizes smaller and less expensive components.
In accordance with the present invention, a method is provided for controlling a three phase, AC induction motor having three or more windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of initially firing each thyristor switch to bring the AC induction motor toward full operating speed at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source. A new alpha firing angle is calculated and each thyristor switch is sequentially fired at the new alpha firing angle after occurrence of zero volts supplied by the AC input source to bring the AC induction motor toward full operating speed. Thereafter, each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough to bring the AC induction motor toward full operating speed.
It is contemplated that the method includes the additional steps of providing the new alpha firing angle as the initial alpha firing angle and repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch. The new alpha firing angle may be calculated according to the expression:
xcex1i=xe2x88x92s*t+k
wherein xcex1i is the new alpha firing angle; s and k are constants; and t is the elapsed time from the initial firing of each thyristor switch.
Upon completion of the predetermined time period, each thyristor switch is sequentially fired at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source. The step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated for a second predetermined time period. Alternatively, motor current in the AC induction motor is monitored and if the motor current is greater than a predetermined value, the step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated
It is contemplated to monitor motor current in the AC induction motor and if the motor current is greater than a predetermined value, execute the additional steps of providing the new alpha firing angle as the initial alpha firing angle and repeating the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch at the new alpha firing angle. The new alpha firing angle is calculated according to the expression:
xcex1i=xcex1ixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex1i is the new firing angle; xcex1ixe2x88x921 is the initial firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and Ilim is a user selected pre-set current limit. However, the absolute value of the expression k*(Ixe2x88x92Ilim) is compared to a limit prior to calculating the new alpha firing angle and replaced with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the limit.
The operating speed of the AC induction motor may be monitored and, if the operating speed is less than the full speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed. The new gamma firing angle is calculated according to the expression:
xcex3i=Yixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex3i is the new gamma firing angle; xcex3ixe2x88x921 is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and Ilim is a user selected pre-set current limit. The absolute value of the expression k*(Ixe2x88x92Ilim) is compared to a gamma limit prior to calculating the new gamma firing angle and is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the gamma limit.
Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed.
In accordance with a further aspect of the present invention, a method is provided for controlling a three phase, AC induction motor having at least three windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of firing each thyristor switch at an alpha firing angle after occurrence of zero volts supplied by the AC input source. The alpha firing angle is sequentially reduced and each thyristor switch is repeatedly fired at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source. The firing of each thyristor switch at the reduced alpha firing angles is stopped and, thereafter, each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough.
Prior to stopping the firing of each thyristor switch, the additional steps of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a predetermined time period are performed Alternatively, the motor current in the AC induction motor is monitored and, if the motor current is greater than a predetermined value, the step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated.
The alpha firing angle is calculated according to the expression:
xcex1i=xcex1ixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex1i is the calculated alpha firing angle; xcex1ixe2x88x921 is the alpha firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and Ilim is a user selected pre-set current limit. However, the absolute value of the expression k*(Ixe2x88x92Ilim) is compared to a limit prior to calculating the new alpha firing angle and replaced with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the limit.
The operating speed of the AC induction motor may be monitored and, if the operating speed is less than the full speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed. The new gamma firing angle is calculated according to the expression:
xcex3i=xcex3ixe2x88x92+k*(Ixe2x88x92Ilim)
wherein xcex3i is the new gamma firing angle; xcex3ixe2x88x921 is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and Ilim is a user selected pre-set current limit. The absolute value of the expression k*(Ixe2x88x92Ilim) is compared to a gamma limit prior to calculating the new gamma firing angle and is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the gamma limit.
Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed.
In accordance with a still further aspect of the present invention, a method is provided for controlling a three phase, AC induction motor having at least three windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of firing initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source. A new alpha firing angle is calculated and each thyristor switch is sequentially fired at the new alpha firing angle after occurrence of zero volts supplied by the AC input source. Thereafter, each thyristor switch is sequentially fired at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source, and then each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough. Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed. If the operating speed is less than the operating speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed.
It is contemplated to provide the new alpha firing angle as the initial alpha firing angle and to repeat, for a predetermined time period, the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch. Upon completion of the predetermined time period, the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source is performed and the step of sequentially firing each thyristor switch at a user selected alpha firing angle is performed for a second predetermined time period.